Transient formation of the phosphoprotein during autophosphorylation of rat mammary gland Golgi vesicles.

Incubation with [gamma-32P] ATP of Golgi vesicles, prepared from the mammary tissue of lactating rats, resulted in the phosphorylation of four of the proteins in the preparation which were resolvable by sodium dodecyl sulphate (SDS)-polyacrylamide gel electrophoresis. Three of these had electrophoretic properties identical to the three major caseins of rat milk: their phosphorylation was approximately linear with respect to time during the course of the short (1 min) incubations analyzed. The fourth component (Mr,app. 70,000) behaved differently. It was very rapidly phosphorylated to a maximum level within 5 S at 0 degree C; its 32 P-content declined thereafter, with a t 1/2 for dephosphorylation of approx. 20 s. The extent of 32P incorporation into this component, measured after incubation for 20 s at 0 degree C with [gamma-32P] ATP, was sensitive to the concentration of Ca2+ in the incubation medium, being enhanced at low concentrations (less than 10-8 M) of Ca2+ and depressed at high (10-4 M) ones. Inclusion of ADP (100 microM) in such incubation also depressed 32P incorporation into the 70 kDa component. This phosphoprotein was further distinguished from the other three by virtue of the lability of its incorporated phosphorus to treatment with hot trichloroacetic acid. The properties and possible function of this phosphoprotein are discussed in relation to the ATP-dependent Ca2+ transport that occurs in this Golgi vesicle preparation.